The present invention relates generally to data transmission in mobile communication systems and more specifically to useful resource block reference signal patterns as well as systems and method for using the patterns.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices or other User Equipment (“UE”) that have telecommunications capabilities. In some embodiments, a UA may refer to a mobile, wireless device. The term “UA” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” or “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UA with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UAs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UAs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UAs, deciding the transport channel to be used for each UA's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UAs through a scheduling channel.
In LTE systems, data is transmitted from an access device to UAs via Resource Blocks (RBs). Referring to FIG. 1, an exemplary resource block 50 is illustrated that is comprised of 168 Resource Elements (REs) (see exemplary elements 52) arranged in twelve frequency columns and fourteen time rows as known in the art. Accordingly, each element corresponds to a different time/frequency combination. The combination of elements in each time row are referred to as an Orthogonal Frequency Division Multiplexing (OFDM) symbol. In the illustrated example the first three OFDM symbols (in some cases it may be the first two, first four, etc.) are reserved for PDCCH and are shown in FIG. 2 as gray REs collectively identified by numeral 56. Various types of data can be communicated in each RE.
LTE systems employ various types of reference signals to facilitate communication between an access device and a UA. A reference signal can be used for several purposes including, determining which of several different communication modes should be used to communicate with UAs, channel estimation, coherent demodulation, channel quality measurement, signal strength measurement, etc. Reference signals are generated based on data known to both an access device and a UA, and may also be referred to as pilot, preamble, training signals, or sounding signals. Exemplary reference signals include a cell specific or common reference signal (CRS) that is sent by a base station to UAs within a cell and is used for channel estimating and channel quality measurement, a UA-specific or dedicated reference signal (DRS) that is sent by a base station to a specific UA within a cell that is used for demodulation of a downlink, a sounding reference signal (SRS) sent by a UA that is used by a base station for channel estimation and channel quality measurement and a demodulation reference signal sent by a UA that is used by a base station for demodulation of an uplink transmission from the UA.
In LTE systems, CRS and DRS are transmitted by access devices in RB REs. To this end, see FIG. 2 which shows exemplary CRS (three of which are labeled 52) in vertical, horizontal, left down to right and left up to right hatching for ports 1 through 3 respectively and exemplary DRS in dark REs, three of which are labeled 54. The reference signals allow any UAs communicating with the access device to determine channel characteristics and to attempt to compensate for poor characteristics. The CRS reference signals are base station/cell specific and UA-independent (i.e., are not specifically encoded for particular UAs) and, in at least some cases, are included in all RBs. By comparing the received CRS to known reference signals (i.e., known data), a UA can determine channel characteristics (e.g., a communication quality index, etc.). The difference between the known data and the received signal may be indicative of signal attenuation, path-loss differences, etc.
UAs report channel characteristics back to the access device and the access device then modifies its output (i.e., subsequent REs) to compensate for the channel characteristics. To indicate how signal output is modified, the access device transmits UA specific DRS to each UA. Here again, DRS data is known at the UA and therefore, by analyzing received DRS, UA can determine how the access device output has been modified and hence obtain information required to demodulate data received in subsequent REs. In FIG. 2, exemplary CRS reference signals are indicated by hatching, DRS signals are indicated by dark REs and non-reference signal elements during which traffic data is transmitted are blank (i.e., white).
Referring again to FIG. 2, to avoid collisions LTE system DRS 54 are generally allocated to OFDM symbols separate from those occupied by CRS. Furthermore, DRS 54 are generally allocated away from PDCCH 56. In release 8 LTE devices (hereinafter “Rel-8 devices”), for example, DRS of antenna port 5 may be specified for PDSCH demodulation as shown in FIG. 2. In some cases, CRS 52 on antenna ports 0-3 are distributed on all RBs in the system bandwidth, while DRS 54 on antenna port 5, for example, may only be allocated in RBs assigned to a corresponding UA. When a UA is assigned two or more contiguous RBs, DRS 54 allocation may simply be repeated from one RB 50 to the next.
One contemplated LTE-A requirement is to reach a peak spectrum efficiency of 30 bps/Hz. To fulfill this requirement, the total RE overhead for DRS will likely be limited. As such, in one system implementation satisfying peak spectrum efficiency requirements, a maximum of 24 DRS REs may be allocated to up to 8 antenna ports. As a result, the average number of REs in one RB for each antenna port becomes relatively small. For instance, for 8 antenna ports, 3 REs per RB may be allocated for each of 8 DRS antenna ports. In another instance, for 8 antenna ports, 4 REs per RB may be allocated for each of 4 antenna ports, and 2 REs per RB may be allocated for each of another 4 antenna ports.
In either example, 2-3 REs per RB may be allocated to each of some or all antenna ports. In that case, with so few DRS REs allocated to each UA, channel characteristic estimation is difficult to maintain at reasonable quality and therefore high data rate demodulation is difficult to facilitate.
One method for a UA to improve channel estimation quality is to carry out channel estimation on contiguous RBs when contiguous RBs are assigned to a single UA. In some cases, the DRS pattern for these multiple RBs may be the repeated versions of a single RB pattern. However, as a result of the scarcity of available REs per antenna port in an RB, the DRS may not be well-distributed and, as a result, may not cover resource edges well. Two example DRS patterns are shown in FIGS. 3a and 3b for two RBs. In both FIGS. 3a and 3b, the DRS patterns are repeated in two separate RBs. Also, in both FIGS. 3a and 3b, REs 102 making up part of the DRS are not fully distributed in time. For example, in FIG. 3a, both DRS 102′ are broadcast at exactly the same time and both DRS 102 are broadcast at exactly the same time.
Because DRS 102, 102′ are not fully distributed in time, two problems can occur. First, there may be a problem with power balance among OFDM symbols if reference signals for different antenna ports are multiplexed in Frequency-Division Multiplexing (FDM) and Time-Division Multiplexing (TDM) fashion. Second, the edge of the assigned resource may not be covered well and extrapolation will therefore be needed for channel estimation, which may cause performance loss. In FIG. 3a, for example, four OFDM RE symbols are located at the edge of the RBs and would require extrapolation.
Therefore, a problem with existing reference signal design is the trade-off between channel estimation quality and overhead where multi-layer transmission is to be supported.